1. Field of the Invention
The present invention relates to a method for manufacturing an electronic micromodule, and more particularly a contactless micromodule comprising an integrated circuit and an antenna coil electrically connected to the integrated circuit.
2. Description of the Related Art
An integrated circuit usually comes in the form of a silicon chip which comprises an active region implanted in the silicon, and contact pads electrically linked to the active region. Some integrated circuits called contactless, like PICC circuits (Proximity Inductive Coupling Circuit) described by the ISO/IEC 14443 and 15693 standards, are intended to be connected to an antenna coil which is the interface between the integrated circuit and the external environment, for receiving and sending data. These integrated circuits comprise means for receiving and/or sending data by inductive coupling by means of the antenna coil, in presence of a magnetic field emitted by a station for sending and/or receiving data. They are used for the manufacture of contactless electronic micromodules which are fixed on or in portable objects like plastic cards, tokens, keys, books, etc, in order to authenticate and/or identify the portable objects.
A manufacturing method often used to make a contactless micromodule consists in providing a support wafer in paper or in plastic on which the antenna coil is made by screen printing a conducting paste or etching a conducting layer. The integrated circuit is then laid down on the support wafer, and its contact pads are connected to the coil.
However, this method imposes technological constraints concerning the thickness and the surface of the micromodule. Thus, the total thickness of the support wafer and of the antenna coil cannot be reduced beyond a minimum of about 100 μm. Equally, the technological pitch (minimum distance between conductors) offered by the techniques of creeping or etching a paper or plastic support imposes a relatively big size of coil clearly superior to the size of the integrated circuit.
Another manufacturing method called “coil on chip” is also known and consists in making coils directly on integrated circuits whereas they are still present on a silicon wafer, before cutting the wafer into silicon chips. This method consists of depositing an insulating layer onto the active face of the silicon wafer (face comprising the active regions of the integrated circuits) and forming the coils on the superior insulating layer. After cutting the silicon wafer, integrated micromodules are obtained.
This method allows several assembly and connection steps to be suppressed but proves to be little reliable and not to have an optimum yield: numerous micromodules thus obtained do not operate properly and must be discarded. This problem results in particular from the fact that the active face of the integrated circuit cannot be flat, particularly because of the presence of conducting elements under the insulating layer. The result is that the thickness of the insulating layer between the coil and the active surface of the integrated circuit is not constant and is sometimes insufficient to avoid the apparition of capacitive or inductive coupling between the integrated circuit and the coil. In addition, openings are made facing the contact pads of the integrated circuit into the insulating layer to ensure the electrical and mechanical contact of the integrated circuit with the antenna. It results in significant irregularity at the surface of the insulating layer (“stairs”) which may also cause electrical discontinuities in the conducting material forming the coil (not enough conducting material on the sides of the stairs). In addition, not properly polymerised dielectrics change with time and do not allow a stable adhesion of the metal. At last, bad cleaning after etching the metal can have a corrosive effect on the conductor and cause a cut of the conducting path.